1. Field of the Invention
The present invention generally relates to an image forming apparatus, and more particularly to an image forming apparatus including a position detector capable of detecting a relative position of and the distance traveled by a movable component and/or a rotational component.
2. Discussion of the Background
In general, image forming apparatuses that employ electrophotographic printing methods, such as copying machines, printers, facsimile machines, etc., include an image forming mechanism and movable elements, such as a photoconductor drum, a transfer belt, and/or a conveyance member. For example, in one electrophotographic printing method, the image forming mechanism forms an electrostatic latent image on a rotating photoconductor drum, develops it with toner, and transfers the developed image onto a recording sheet. The recording sheet is conveyed to the image forming mechanism by a conveyance belt.
A color image may be formed by superimposing images of different colors on top of one another. Images of different colors on a recording sheet or a transfer member may be displaced relative to each other when being superimposed, which blurs the color image. Therefore, positioning of the images is important to avoid position displacement among the different colors of a color image. Positioning may require detection of position, distance traveled, and/or speed of a movable element.
One type of image forming apparatus includes an image sensor to measure positional displacement. The image sensor reads an image on a movable part, compares the current image data with a previously read image data, and computes the position of the image. The above method is convenient since it requires only a detector and a reference scale is not required. However, extensive calculations may be required to accurately calculate the position or distance traveled by the image using only an image sensor.
One type of position detector may employ a method in which a correlation coefficient of density data of an optical mark is calculated and a position having a highest correlation is regarded as a relative displacement position. A large amount of calculation may be required because a correlative coefficient regarding every pixel is calculated.
Another type of image forming apparatus is provided with an encoder including a scale having a pattern in which optical marks are arranged at substantially constant intervals to detect and to measure the distance traveled by a movable element. The encoder detects position and/or speed of the movable element, measures them using the scale, and converts the measured values into electrical signals. However, to accurately calculate a relative position or distance traveled by a movable element, a highly accurate optical system and high quality scale and sensor are required. Furthermore, when optical marks are provided on a flexible or deformable component, variation may be caused in the intervals due to environmental factors, such as temperature and/or humidity.